Controlling and monitoring combustible gases

ABSTRACT

Apparatus for monitoring and controlling the flow of a combustible gas includes a solenoid operated gas valve to control the gas flow. The solenoid is energized from an AC supply via a thyristor switch. When the gas is ignited an ionization current flows through the flame and is monitored to control the trigger electrode of the thyristor. When the flame is out the thyristor is not triggered so that it remains cut-off and deenergizes the solenoid. This closes the gas valve to cut off the gas flow. Triggering may take place through a phase advance circuit. Ignition sparks may be provided by means of a capacitor and a step-up transformer arrangement in which the capacitor is periodically discharged through the thyristor to produce spark pulses via the transformer.

ilnited States Patent 1191 Gray [ CONTROLLING AND MONITORING COMBUSTIBLE GASES [75] Inventor: Stuart William Gray, Sutton, Surry,

England [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: May 8, 1970 [21] Appl. No.: 35,852

[30] Foreign Application Priority Data May 9, 1969 Great Britain 23,763/69 [52] US. Cl. 317/157, 317/148.5 B, 317/151, 3l7/DIG. 1, 323/22 SC, 323/36, IMO/228.1,

[51] Int. Cl. F23n 5/00 [58] Field of Search 431/78, 71, 25, 6,

431/24; 3l7/148.5 B, 151, DIG. 1; 340/228.1; 323/19, 22 SC, 38, 36

[56] References Cited UNITED STATES PATENTS 3,348,104 10/1967 Zielinski et a1. 431/78 UX 3,377,125 4/1968 Zielinski 431/74 3,270,799 9/1966 Pinckaers 431/25 [11] 3,76,441' [451 Oct. 16, 1973 FOREIGN PATENTS OR APPLICATIONS 1,181,965 2/1970 Great Britain 431/78 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney-Frank R. Trifari [5 7] ABSTRACT Apparatus for monitoring and controlling the flow of a combustible gas includes a solenoid operated gas valve to control the gas flow. The solenoid is energized from an AC supply via a thyristor switch. When the gas is ignited an ionization current flows through the flame and is monitored to control the trigger electrode of the thyristor. When the flame is out the thyristor is not triggered so that it remains cut-off and deenergizes the solenoid. This closes the gas valve to cut off the gas flow. Triggering may take place through a phase advance circuit. Ignition sparks may be provided by means of a capacitor and a step-up transformer arrangement in which the capacitor is periodically discharged through the thyristor to produce sparkpulses via the transformer.

19 Claims, 1 Drawing Figure SCR I CONTROLLING AND MONITORING COMBUSTIBLE GASES The present invention relates to a method of and apparatus for controlling and monitoring a combustible gas. The invention also relates to a method of and apparatus for igniting the combustible gas.

Where it is desired to automatically ignite a conbustible gas such as a natural or artifically produced hydrocarbon gas it is necessary to have some means of monitoring whether or not the gas is in fact ignited and to cut off the supply of gas if the gas isnot ignited. Such monitoring and control is necessary in gas fired central heating appliances.

It is an object of the present invention to provide a method of and apparatus for monitoring and controlling a combustible gas employing electronic techniques.

The present invention provides a method of monitoring and controlling a combustible gas, in which method a solenoid operable gas valve is provided which, when opened, allows a stream of said gasto pass to a burner. The solenoid is energisable by current flowing through a semiconductor switch when in its conductive state. An ionisation current which flows through the flame of the gas when ignited at the burner allows the switch to be triggered to render it conductive whereas in the ab sence of the flame the switch is prevented from being triggered to hold it non-conducting thereby closing the gas valve to cut off the gas stream to the burner.

Such a method may also incorporate a method of igniting the gas at the burner, in which ignition method a previously charged capacitor is periodically discharged through the switch to produce a train of sparks at the burner. The switch at these periods is rendered conductive thereby energising the solenoid to allow the gas stream to pass to the burner. The train of sparks may only be produced for a limited period, after which, if the gas is ignited, flame ionisation current causes the switch to be triggered, or if the gas is not ignited, the switch is prevented from being triggered.

' The invention also provides apparatus for carrying out the method of the present invention. In such apparatus the solenoid operable gas valve may be energisable from an alternating supply. When the gas is ignited, the switch is triggered, at or near the beginning of each cycle of the alternating supply. Triggering takes place through a phase advance circuit. The solenoid of the gas valve and the switch may be connected in series across the alternating supply. The semiconductor switch may be a thyristor and a diode may be provided in the series connection betwen the solenoid and the thyristor, the diode being poled for conduction in the same direction as the thyristor. The semiconductor switch may alternatively be a triac, i.e. a bi-directional semiconductor switch, and a diode may be provided in I the series connection between the solenoid and the triac.

The flame ionisation current may be applied to the base of a first transistor to render that transistor nonconducting during at least a portion of each cycle of the alternating supply, at least a part of this portion coinciding with the beginning of each cycle when the switch is to be triggered. In the absence of the flame ionisation current the first transistor is rendered conductive to prevent the switch from being triggered when the first transistormay be rendered conductive by a bias applied to its base derived from the alternating supply.

The first transistor may be provided in cascade with second and third transistors, the third transistor being rendered conductive when the first transistor is conductive.

When the flame ionisation current is available, the switch may be triggered from a resistor-capacitor circuit connected between one side of the alternating supply and the collector of the third transistor, the collector also being connected to the trigger electrode of the switch. The emitter of the third transistor may be connected to the other side of the alternating supply to which the electrode of the switch remote from the solenoid also is connected. In the absence of this flame ionisation current the third transistor is rendered conductive bypassing the triggering current from the trigger electrode of the switch.

The invention also provides apparatus for igniting a combustible gas in which a capacitor is serially connected with the primary of a transformer, the resulting series circuit being connected in parallel with the switch. The capacitor is charged from the alternating supply from the commencement of each cycle of the supply for a period until the switch is triggered. The switch, when periodically triggered, discharges the capacitor through the primary of the transformer to produce atrain of sparks across a spark gap located at the burner and connected across the secondary winding of the transformer. During the ignition period, the thyristor is triggered at between 30 and after the commencement of each cycle.

The train of sparks may be produced during a period commencing at or just after the energisation of the apparatus from the alternating supply and ending when the charge on a second capacitor in the bias circuit for the base of the first transistor has reached a given level. After this period the switch may be triggered at or near the commencement of each cycle if the gas has ignited, or if the gas has not ignited, the switch may be prevented from being triggered. If the gas has not been ignited, the train of sparks may not be reproduced until the apparatus has been disconnected and subsequently reconnected to the alternating supply. Alternatively, a, switch may be provided for discharging the second capacitor. After discharging the second capacitor, a second period maybe provided during which a further train of sparks may be produced and the solenoid reenergised.

The above and other features of the present invention will be more readily understood by a perusal of the following description having reference to the accompanying drawing which shows a circuit arrangement of apparatus for the method of the present invention.

Referring now to the sole FIGURE of the accompanying drawing there is shown a circuit arrangement capable of monitoring, controlling and igniting a stream of combustible hydrocarbon gas. The circuit is arranged to be connected between the neutral N and live L terminals of an alternating supply such as of 240 volts 50 Hz. A solenoid operated gas valve V is provided which, when energized, allows a stream of the gas to pass to a gas burner B. The solenoid of the gas valve V is provided between the neutral and live supply lines in series with a diode D4 and a thyristor SCR, both of which are poled for conduction in the same direction, i.e. when the neutral line N is positive with respect to 3 the live line L. Thyristor SCR is shunted by a series circuit formed by a capacitor C4 and the primary winding of a transformer T, the secondary winding of this transformer having its terminals connected to the gas burner B, which is at earth potential, and an ignition electrode 1 which forms a spark gap. A further electrode M provided adjacent to the burner B is returned through a resistor R3 to the base of an npn transistor TRl whose emitter is directly connected to the supply line L. The base of transistor TRl is also coupled to the supply line L through a capacitor C2 and is additionally connected to the junction of a resistor R1 and a capacitor C1 through a further resistor R2. The terminal of capacitor C1 remote from this junction is connected to the supply line L while the terminal of resistor R1 remote from the junction is connected through a diode D1 to the junction of a pair of serially connected resistors R6, R7. The collector of transistor TRl is connected through a resistor R4 to the supply line N and is directly connected to the base of a further npn transistor TR2 and through a diode D2 to the supply line L. The emitter electrode of transistor TR2 is directly connected to the supply line L while its collector electrode is connected through a resistor R5 to the junction of the serially connected resistors R6, R7. Additionally the collector of transistor TR2 is directly connected to the base of a npn transistor TR3 whose emitter is directly connected to the supply line ,L. The collector electrode of the latter mentioned transistor is connected to the terminal of resistor R7 remote from its junction with resistor R6. The terminal of resistor R6 remote from the junction is connected through a capacitor C3 to the supply line N, the junction being additionally connected through a diode D3 to the supply line L. In addition the collector electrode of transistor TR3 is directly connected to the trigger electrode of the thyristor SCR.

' In considering the operation of the circuit arrangement it is assumed that initially the terminals L and N are disconnected from the alternating supply in which case no part of the circuit will be operative and gas valve V will be closed due to the solenoid being deenergised which in turn prevents a stream of gas from being passed totheburner B. On application of the alternating supply-to the terminals N and L, a phase advance circuit is formed by capacitor C3 and resistors R6, R7 phase advancing the current applied to'thetrigger electrode of thyristor SCR by nearly 90with respect to its anode voltage and ensuring adequate current to trigger the thyristor'at the beginning of each positive half cycle at the supply line N. This causes the solenoid of gas valve V to be energized thereby opening the valve and allowing a stream of gas to be passed to the burner B. At the initial application of the AC supply to the circuit, capacitor C1 is in a discharged state and since its charge path includes resistor R1, which has a high value (in the order of l M ohm) it takes a few seconds for this capacitor to be charged from the direct voltage supply produced by diode D1 from the junction of resistors R6, R7. Thus transistor TRl is initially cut off allowing transistor TR2 to conduct'during positive half cycles at the supply line N as this latter transistor is biased through resistor R4. Driving negative half cycles the base of transistor TR2 is connected to the line L through the diode D2 cutting off transistor TR2 and additionally preventing the collector of transistor TRl from going negative. During the positive half cycles transistor TR3 is non-conducting causing the phase advanced triggering pulses to be applied to the trigger electrode of thyristor SCR. As capacitor Cl is gr u= ally charged, transistor PR1 gradually turns on progres= sively diverting current through resistor R4 away from the .base of transistor TR2. Due to the sinuosidal cur= rent through resistor R4 transistor TR2 is turned on progressively later in each half cycle thus causing the thyristor SCR to be triggered later in each half cycle, i.e. somewhere between 30 and after the com mencernent of each positive half cycle, the solenoid oi gas valve V remaining energised if triggering takes place before 90. When the thyristor SCR is triggered later in the cycle, say at 30, the capacitor can be charged from the alternating supply. This capacitor is discharged through the primary of transformer T when the thyristor SCR is triggered and rendered conductive. The transformer T receives the energy stored in the ca= pacitor C4 and dissipates it across the spark gas formed between the ignition electrode 1 and the burner B, these sparks providing ignition for the gas stream. if the gas is ignited and a flame is produced at the burner is an al= ternating current flows from the burner 12, which is at or near neutral potential, by way of theionised gas in the flame, the monitorin electrode M and resistor R3. This alternating current is phase retarded by the capacitor C2 and a lied to the base of transistor T'Ri. Dur= ing a portion of each alternating cycle this alternating current opposes the bias current applied to the base of transistor TRl to render that transistor non= conducting. Due to the fact that the alternating current from the monitoring electrode M is phase ifeta'ttiled transistor TRl remains non=conducting for a major portion of each cycle. Part of this non conducting por tion coincides with the commencement of each positive half cycle thereby rendering,during this time, transis= tors TR2 and TR3 conducting and 'nomconducting, re

spectively, and causing thyristor SCR to be turned on at the beginning of each cycle when the flame is burn= ing. With the circuit in this condition capacitor C4 is prevented from being charged and sparks will not occur at the spark gap.

, If, however, the as fails to ignite in the time-it takes to nearly fully charge capacitor C1, or if the flame is at any time extinguished, transistor TRl and hence tram sistor TR3 are held conductive thereby preventing triggering pulses from beingapplied to the gate electrode of the thyristor SCR. This in turn causesgas valve V to be closed. At this time capacitor C4 is charged but as.

thyristor SCR is not rendered conductive, a discharge path is not provided for the capacitor and sparks will not be produced at the spark gap. The circuitwill then be locked out and in orderto return it to the igniting condition it is necessary to disconnect the circuit arrangement from the alternating supply for a matter of several seconds. Alternatively, a switch may be provided across capacitor C1 capable of discharging that.

capacitor after which time the charge operation 'will recommence and allow a train of sparks to be produced at the burner B and the solenoid to be re-energized.

The function of diode D3 in the circuit is to provide a reverse current path for capacitor C3 and resistor R6,-

preventing the junction of resistor R6, R7 from going negative and so preventing the collectors of transistors TR2 and TR3 from going negative with respect to their emitters.

In a practical arrangement of the circuit, the following components were employed:

R1 1 M ohm R2 10 M ohm R3 10 M ohm R4 10 M ohm R5 100 K ohm R6 1.2 K ohm R7 1.2 K ohm Cl l p. Farad .C2 3.3 n Farad C3 0.15 p, Farad C4 0.47 y. Farad TR] Mullard Limited Type BC 149 TR2 Mullard Limited Type BC 148 TR3 Mullard Limited Type BC 148 D1 Mullard Limited Type BAX 13 D2 Mullard Limited Type BAX 13 D3 Mullard Limited Type A 91 D4 Mullard Limited Type BY 126 SCR Mullard Limited Type BT 100A T Primary 25 turns, secondary 1,250 turns.

I claim:

1. Apparatus for monitoring and controlling the flow of a combustible gas comprising, an alternating current supply, a gas'valve operated by a solenoid for controlling the flow of gas to a burner, a semiconductor thyristor switch having a control electrode, means connecting the switch in series circuit with the solenoid and the AC supply so that the switch controls the flow of current from the AC supply to the solenoid, means for monitoring the ionization current that flows through the burner gas flame when the gas is ignited, circuit means coupling said monitoring means to said control electrode and responsive thereto so as to trigget the switch into conduction when the gas is ignited and to inhibit the triggering thereof in the absence of a flame whereby-the solenoid is operated to close the gas valve and cut off the gas flow to the burner, and said circuit means includes a phase advance circuit connecting said control electrode to the AC supply thereby to trigger said thyristor switch from said AC supply. 1

2. Apparatus as claimed in claim 1 wherein said circuit means further comprises a first transistor coupled to the control electrode of said thyristor switch to inhibit the triggering thereof whensaid first transistor is conductive, means for applying to the base of said first transistor a bias voltage derived from said AC supply and of a polarity to produce conduction in the transistor, means for applying the flame ionisation current to the base of said first transistor with a phase retardation relative to the AC supply so as to render that transistor non-conducting during at least a portion of each cycle of said alternating supply, at least a part of said portion coinciding with the beginning of each positive half cycle of the AC supply thereby allowing said switch to be triggered. I r

3. Apparatus as claimed in claim 2 in which, in the absence of said flame ionisation current, said first transistor is rendered conductive by the bias voltage applied 'to its base thereby to inhibit the triggering of said thyristor switch.

4. Apparatus as claimed in claim 2 wherein said circuit means further comprises second and third transistors, means operatively connecting said first transistor in cascade with said second and third transistors so that said third transistor is rendered conductive when said v first transistor is conductive, and means connecting conduction in the absence of said flame ionization cursaid third transistor to the thyristor control electrode.

5. Apparatus as claimed in claim 4 wherein said circuit means further comprises a resistor capacitor circuit connected between one side of said alternating supply and the collector of said third transistor so that, with the flame ionization current present, the thyristor switch is triggered via said RC circuit, the collector of said third transistor also being connected to the trigger electrode of said switch.

6. Apparatus as claimed in claim 5 in which said third transistor is connected between the cathode and the trigger electrode of said thyristor switch so that, in the absence of said flame ionisation current, said third transistor is rendered conductive bypassing the triggering current away from the trigger electrode of said switch.

7. Apparatus for monitoring and controlling the flow of a combustible gas comprising, an alternating current supply, a gas valve operated by a solenoid for controlling the flow of gas to a burner, a semiconductor switch having a control electrode, means connecting the switchin series circuit with the solenoid and the AC supply so that the switch controls the flow of current from the AC supply to the solenoid, means for monitoring the ionization current that flows through the burner gas flame when the gas is ignited, circuit means coupling said monitoring means to said control electrode and and responsive thereto so as to trigger the switch into conduction when the gas is ignited and to inhibit the triggering thereof in the absence of a flame whereby the solenoid is operated to close the gas valve and cutoff the gas flow to the burner, a diode and a capacitor serially connected with the primary winding of a transformer across said AC supply, and means connecting the series circuit comprising said capacitor and primary winding in parallel with said switch so that said capacitor is charged from said'alternating supply from the commencement of each cycle of said AC supply for a period until said switch is triggered and then is discharged via said switch through theprimary of said transformer to produce a train of sparks across a spark gap located at said burner and connected across the secondary winding of said transformer.

8. Apparatus as claimed in'claim 1 further comprising a capacitor connected in series circuit with the primary winding of an ignition transformer across said AC supply so as to charge the capacitor at the start of each cycle of said AC supply until the switch is triggered into conduction, means connecting said series circuit in parallel with said switch so that the switch provides a discharge path for the capacitor through said primary winding,and wherein said circuit means includes electric delay means which, during a burner ignition period, delays the triggering of said switch to a period between 30 and subsequent to the start of each cycle of said AC supply, and means connecting the secondary winding of said transformer to an ignition electrode of the burner.

9. Apparatus as claimed in claim 8 wherein said circuit means further comprises a first transistor coupled to the control electrode of said semiconductor switch so as to inhibit the triggering thereof when said first transistor is fully conductive, and a bias capacitor coupled to the base of the second transistor and to said AC supply and arranged to bias said first transistor into full rent.

10. Apparatus as claimed in claim 9vwherein said circuit means includes means for coupling the flame ionization current, when present, from said monitoring means to the base of the first transistor so as cut-off said first transistor at the start of each cycle of said AC supply thereby to allow said semiconductor switch to be triggered into conduction at that time.

11. Apparatus as claimed in claim 1 wherein said circuit means further comprises a phase delay circuit interposed between said monitoring means and said control electrode whereby the triggering of the switch is controlled jointly by the AC supply and the ionisation current at a time subsequent to ignition of the gas.

12. Flow control apparatus for a combustible gas comprising, input terminals adapted for connection to a source of AC supply current, a solenoid operated gas valve for controlling the gas flow, a semiconductor controlled switch connected in series circuit with the input terminals and the solenoid so that the switch controls the fiow of current from the input terminals to the solenoid, phase control means coupling an input terminal to a control electrode of said switch to supply a periodic trigger signal thereto in advance of the start of each cycle of the AC supply and independent of the state of the gas thereby to initiate conduction through said switch, means for monitoring the gas so as to derive an ionization current from the gas when the gas is burning, a first current control element with its output coupled to the control electrode of the switch and its input coupled to said monitoring'means so as to respond to said ionization current, when present, said control element being operative to apply a first control signal to said control electrode that will inhibit the triggering of the switch when said ionization current is absent at its input thereby to operate the solenoid to close off the gas valve and being operative when said ionization current is present at its input to apply .a second control signal thereto at or near the start of each cycle of the AC supply that allows said trigger signal to then trigger the switch into conduction.

l3. Apparatus'as claimed in claim 12 wherein the input circuit of said control element further comprises means coupled to the input terminals for varying the DC bias voltage applied'to the control element during an initial ignition period for the apparatus such that, in the absence of the ionization current said first control signal is varied as a function of the bias voltagein a manner such as to gradually vary the delay inthe triggering of the switch relative to the start of each cycle of the AC supply. a 7

14. Apparatus as claimed in claim 12 further. comprising a capacitor connected in series circuit with the primary winding of an ignition transformer, means connecting said series circuit in parallel with said switch so that the switch provides a path for selectively discharging the capacitor through said primary winding, a charge circuit coupling said series. circuit to the input terminals so that the capacitor is charged from said AC supply when the switch is cut-off, and means connecting the-secondary winding'of the transformer to an ignition electrode whereby sparks are produced at said electrode in synchronism with the discharge of the capacitor.

15. Apparatus as claimed in claim 14 further comprising a bias capacitor connected to the input of said first control element and to the input terminals via a DC charge circuit and operative when the charge thereon reaches a given level to apply a bias voltage to said control element which, in the absence of any ionization current, will cause said control element to derive said first inhibit control signal to prevent the triggering of the switch and hence prevent the generation of sparks.

16. Apparatus as claimed in claim 15 further comprising means for retarding the phase of said AC ionization current relative to the AC supply so as to overide said bias voltage near the start of each cycle of the AC supply whereby said control element becomes operative to derive and apply said second control signal to the switch to trigger same near the start of each cycle of the AC supply and thereby prevent the first capacitor from charging to a level sufficient to generate sparks.

17. Apparatus as claimed in claim 16 wherein said charge circuit includes a rectifier element, said switch comprises a thyristor, and said phase control means includes a capacitor for advancing the phase of the trigger signal relative to the AC supply.

18. Flow control apparatus for a combustible gas comprisingflnput terminals adapted for connection to a source of AC supply current, a solenoid operated gas valve for controlling the gas flow, a semiconductor controlled switch connected in series circuit with the solenoid-across the input terminals to control the current flow from said terminals to the solenoid, phase control means coupling an input terminal to a control electrode of said switch to supply a periodic trigger signal thereto in advance of the positive half cycle of the AC supply voltage applied to the anode of the switch, ignition means for igniting said gas and coupledto said switch, means for monitoring the gas so as to derive an electric signal therefrom when the gas is ignited, and circuit means responsive to said signal, when present, to apply a first control signal to said control electrode at or near the start of each positive half cycle of the AC supply which allows said trigger signal to trigger the switch into conduction and thereby inhibits the operation of I said ignition means, said circuit means being operative to delay the triggering of said switch for a given ignition period when said signal is absent thereby to operate said ignition means via saidswitch and to subsequently apply a second control signal to said control electrode that-will inhibit the triggering of the switch if the gas does not ignite during said ignition period.

19. Apparatus as claimed in claim 18'wherein said ignition means comprises a capacitor and an ignition coil serially connected across the switch terminals and to the input terminals in a manner whereby said capacitor is allowed to charge up only when the switch is cutoff, and wherein said circuit means includes an amplifier with a control electrode coupled to said monitoring means and an output electrode coupled to the control electrode of the semiconductor switch. I

Po-ww UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION pa 3,766,441 Dated October 16, 1973 STUART WILLIAM GRAY Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r" v a g 1 col. 1, line 10, after "gas" insert a comma line 20, cancel the comma cancel "method"; lines 25-26, cancel "flame of the gas" and insert gas flame line 27, after "conductive" insert a comma 7 lines 46 48,5l.,63,after "alternating" insert current .col. 2 line 15, 'cancel' 'this" and insert the line 31 after "at" insert a point line 60, after "alternating" insert ---current cancel "of" (last occurrence) after; "volts" insert a comma 7 h col. 3, line 3, cancel the" and insert T1fie line 4, cancel "having" and insert has line 40, cancel "and" and insert The line 44, before -"supply" insert 'current L 5 3 UNITED STATES "PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 7 441 I Dated O t b 16 1,973

lnventor(s) STUART WILLIAM GRAY It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

col. 3, line 6 7 cancel "causing" and insert allowing col. 4, line 62, change "resistor" to resistors In the claims, col. 6, line 3, after "resistor" insert a hQ'phen I line 28, cancel "and" (2nd occurrence) line 64, cancel "second" and insert first col. 8, line 13, cancel "AC" Lines 41 & 48, before "signal" insert electric Signed and sealed this 11ml day of Ma iQYLL;

(SEAL)- Attest:

EDI-JAR?) MJLETCHERJR. C. MARSHALL DA'NN Atte sting Officer Commissioner of Patents 

1. Apparatus for monitoring and controlling the flow of a combustible gas comprising, an alternating current supply, a gas valve operated by a solenoid for controlling the flow of gas to a burner, a semiconductor thyristor switch having a control electrode, means connecting the switch in series circuit with the solenoid and the AC supply so that the switch controls the flow of current from the AC supply to the solenoid, means for monitoring the ionization current that flows through the burner gas flame when the gas is ignited, circuit means coupling said monitoring means to said control electrode and responsive thereto so as to trigger the switch into conduction when the gas is ignited and to inhibit the triggering thereof in the absence of a flame whereby the solenoid is operated to close the gas valve and cut off the gas flow to the burner, and said circuit means includes a phase advance circuit connecting said control electrode to the AC supply thereby to trigger said thyristor switch from said AC supply.
 2. Apparatus as claimed in claim 1 wherein said circuit means further comprises a first transistor coupled to the control electrode of said thyristor switch to inhibit the triggering thereof when said first transistor is conductive, means for applying to the base of said first transistor a bias voltage derived from said AC supply and of a polarity to produce conduction in the transistor, means for applying the flame ionisation current to the base of said first transistor with a phase retardation relative to the AC supply so as to render that transistor non-conducting during at least a portion of each cycle of said alternating supply, at least a part of said portion coinciding with the beginning of each positive half cycle of the AC supply thereby allowing said switch to be triggered.
 3. Apparatus as claimed in claim 2 in which, in the absence of said flame ionisation current, said first transistor is rendered conductive by the bias voltage applied to its base thereby to inhibit the triggering of said thyristor switch.
 4. Apparatus as claimed in claim 2 wherein said circuit means further comprises second and third transistors, means operatively connecting said first transistor in cascade with said second and third transistors so that said third transistor is rendered conductive when said first transistor is conductive, and means connecting said third transistor to the thyristor control electrode.
 5. Apparatus as claimed in claim 4 wherein said circuit means further comprises a resistor capacitor circuit connected between one side of said alternating supply and the collector of said third transistor so that, with the flame ionization current present, the thyristor switch is triggered via said RC circuit, the collector of said third transistor also being connected to the trigger electrode of said switch.
 6. Apparatus as claimed in claim 5 in which said third transistor is connected between the cathode and the trigger electrode of said thyristor switch so that, in the absence of said flame ionisation current, said third transistor is rendered conductive bypassing the triggering current away from the trigger electrode of said switch.
 7. Apparatus for monitoring and controlling the flow of a combustible gas comprising, an alternating current supply, a gas valve Operated by a solenoid for controlling the flow of gas to a burner, a semiconductor switch having a control electrode, means connecting the switch in series circuit with the solenoid and the AC supply so that the switch controls the flow of current from the AC supply to the solenoid, means for monitoring the ionization current that flows through the burner gas flame when the gas is ignited, circuit means coupling said monitoring means to said control electrode and and responsive thereto so as to trigger the switch into conduction when the gas is ignited and to inhibit the triggering thereof in the absence of a flame whereby the solenoid is operated to close the gas valve and cut off the gas flow to the burner, a diode and a capacitor serially connected with the primary winding of a transformer across said AC supply, and means connecting the series circuit comprising said capacitor and primary winding in parallel with said switch so that said capacitor is charged from said alternating supply from the commencement of each cycle of said AC supply for a period until said switch is triggered and then is discharged via said switch through the primary of said transformer to produce a train of sparks across a spark gap located at said burner and connected across the secondary winding of said transformer.
 8. Apparatus as claimed in claim 1 further comprising a capacitor connected in series circuit with the primary winding of an ignition transformer across said AC supply so as to charge the capacitor at the start of each cycle of said AC supply until the switch is triggered into conduction, means connecting said series circuit in parallel with said switch so that the switch provides a discharge path for the capacitor through said primary winding, and wherein said circuit means includes electric delay means which, during a burner ignition period, delays the triggering of said switch to a period between 30* and 90* subsequent to the start of each cycle of said AC supply, and means connecting the secondary winding of said transformer to an ignition electrode of the burner.
 9. Apparatus as claimed in claim 8 wherein said circuit means further comprises a first transistor coupled to the control electrode of said semiconductor switch so as to inhibit the triggering thereof when said first transistor is fully conductive, and a bias capacitor coupled to the base of the second transistor and to said AC supply and arranged to bias said first transistor into full conduction in the absence of said flame ionization current.
 10. Apparatus as claimed in claim 9 wherein said circuit means includes means for coupling the flame ionization current, when present, from said monitoring means to the base of the first transistor so as cut-off said first transistor at the start of each cycle of said AC supply thereby to allow said semiconductor switch to be triggered into conduction at that time.
 11. Apparatus as claimed in claim 1 wherein said circuit means further comprises a phase delay circuit interposed between said monitoring means and said control electrode whereby the triggering of the switch is controlled jointly by the AC supply and the ionisation current at a time subsequent to ignition of the gas.
 12. Flow control apparatus for a combustible gas comprising, input terminals adapted for connection to a source of AC supply current, a solenoid operated gas valve for controlling the gas flow, a semiconductor controlled switch connected in series circuit with the input terminals and the solenoid so that the switch controls the flow of current from the input terminals to the solenoid, phase control means coupling an input terminal to a control electrode of said switch to supply a periodic trigger signal thereto in advance of the start of each cycle of the AC supply and independent of the state of the gas thereby to initiate conduction through said switch, means for monitoring the gas so as to derive an ionization current from the gas when the gAs is burning, a first current control element with its output coupled to the control electrode of the switch and its input coupled to said monitoring means so as to respond to said ionization current, when present, said control element being operative to apply a first control signal to said control electrode that will inhibit the triggering of the switch when said ionization current is absent at its input thereby to operate the solenoid to close off the gas valve and being operative when said ionization current is present at its input to apply a second control signal thereto at or near the start of each cycle of the AC supply that allows said trigger signal to then trigger the switch into conduction.
 13. Apparatus as claimed in claim 12 wherein the input circuit of said control element further comprises means coupled to the input terminals for varying the DC bias voltage applied to the control element during an initial ignition period for the apparatus such that, in the absence of the ionization current, said first control signal is varied as a function of the bias voltage in a manner such as to gradually vary the delay in the triggering of the switch relative to the start of each cycle of the AC supply.
 14. Apparatus as claimed in claim 12 further comprising a capacitor connected in series circuit with the primary winding of an ignition transformer, means connecting said series circuit in parallel with said switch so that the switch provides a path for selectively discharging the capacitor through said primary winding, a charge circuit coupling said series circuit to the input terminals so that the capacitor is charged from said AC supply when the switch is cut-off, and means connecting the secondary winding of the transformer to an ignition electrode whereby sparks are produced at said electrode in synchronism with the discharge of the capacitor.
 15. Apparatus as claimed in claim 14 further comprising a bias capacitor connected to the input of said first control element and to the input terminals via a DC charge circuit and operative when the charge thereon reaches a given level to apply a bias voltage to said control element which, in the absence of any ionization current, will cause said control element to derive said first inhibit control signal to prevent the triggering of the switch and hence prevent the generation of sparks.
 16. Apparatus as claimed in claim 15 further comprising means for retarding the phase of said AC ionization current relative to the AC supply so as to overide said bias voltage near the start of each cycle of the AC supply whereby said control element becomes operative to derive and apply said second control signal to the switch to trigger same near the start of each cycle of the AC supply and thereby prevent the first capacitor from charging to a level sufficient to generate sparks.
 17. Apparatus as claimed in claim 16 wherein said charge circuit includes a rectifier element, said switch comprises a thyristor, and said phase control means includes a capacitor for advancing the phase of the trigger signal relative to the AC supply.
 18. Flow control apparatus for a combustible gas comprising, input terminals adapted for connection to a source of AC supply current, a solenoid operated gas valve for controlling the gas flow, a semiconductor controlled switch connected in series circuit with the solenoid across the input terminals to control the current flow from said terminals to the solenoid, phase control means coupling an input terminal to a control electrode of said switch to supply a periodic trigger signal thereto in advance of the positive half cycle of the AC supply voltage applied to the anode of the switch, ignition means for igniting said gas and coupled to said switch, means for monitoring the gas so as to derive an electric signal therefrom when the gas is ignited, and circuit means responsive to said signal, when present, to apply a first control signal to said control electrode at or near the start of each positive half cycle of the AC supply which allows said trigger signal to trigger the switch into conduction and thereby inhibits the operation of said ignition means, said circuit means being operative to delay the triggering of said switch for a given ignition period when said signal is absent thereby to operate said ignition means via said switch and to subsequently apply a second control signal to said control electrode that will inhibit the triggering of the switch if the gas does not ignite during said ignition period.
 19. Apparatus as claimed in claim 18 wherein said ignition means comprises a capacitor and an ignition coil serially connected across the switch terminals and to the input terminals in a manner whereby said capacitor is allowed to charge up only when the switch is cut-off, and wherein said circuit means includes an amplifier with a control electrode coupled to said monitoring means and an output electrode coupled to the control electrode of the semiconductor switch. 